1. Field of Invention
The present invention relates to a display device. More particularly, the present invention relates to a storage capacitor structure.
2. Description of Related Art
Display devices have found widespread usage in our daily life. Television and computer monitors are common display devices that show different kinds of images or motions on a screen. Formerly, cathode ray tubes were widely used. However, due to bulkiness and power consumption, cathode ray tubes cannot be used for portable equipment such as a notebook computer. Nowadays, consumers welcome the newly developed dot matrix type of flat panel displays such as liquid crystal display (LCD) or thin film transistor (TFT) LCD. An array of picture pieces or pixels on the TFT LCD constitutes an image with the switching of each pixel controlled by a thin film transistor.
FIG. 1 is a schematic diagram showing the driving circuit of a conventional thin film transistor liquid crystal display. The TFT LCD requires a scan circuit 100 and a signal-holding circuit 102. The scan circuit 100 drives a group of scan lines 110 and the signal-holding circuit 102 drives a group of signal lines 112. The scan lines 110 and the signal lines 112 cross each other perpendicularly forming a two-dimensional array. Each cross-point in the two-dimensional array has a thin film transistor 104, a storage capacitor 108 and a liquid crystal display (LCD) cell 106. The thin film transistor 104, the storage capacitor 108 and the LCD cell 106 together constitute a pixel. The gate terminal of the thin film transistor 104 is controlled by the corresponding scan line 110 and the source terminal of the thin film transistor 104 is controlled by the corresponding signal line 1112. The drain terminal of the thin film transistor 104 is connected to a pixel electrode layer and an electrode of the storage capacitor 108. The storage capacitor 108 maintains a voltage for controlling the liquid crystal. Another electrode of the storage capacitor 108 is connected to an adjacent scan line.
Following the gradual reduction in dimensional layout of a thin film transistor, a common electrode type of storage capacitor design is selected for reducing the effect of gate-driven delay. In this design, the common electrode and the gate terminal are separated from each other so that the other terminal of the capacitor receives a common voltage such as a common electrode voltage (Vcom).
FIG. 2 is a schematic diagram showing the layout of a unit cell of a conventional thin film transistor liquid crystal display. As shown in FIG. 2, the gate terminal of the thin film transistor 104(g) is connected to the scan line 110. The source terminal of the thin film transistor 104(s) is connected to the corresponding signal line 112. The drain terminal of the thin film transistor 104(d) is connected to a pixel electrode layer 1118. A common lower electrode 114 and an upper electrode 116 together constitute a storage capacitor. The pixel electrode layer 118 and the upper electrode 116 are linked through an opening 120.
The lower electrode 114 is formed on a transparent substrate. The lower electrode 114 made of a first metallic layer is patterned together with the gate terminal of the thin film transistor 104. A capacitor dielectric layer is formed on the lower electrode 114. A metallic electrode layer 116 made of a second metallic layer is formed on the capacitor dielectric layer to serve as an upper electrode for the storage capacitor. The overlapping region between the upper electrode 116 and the lower electrode 114 is the main charge storage area for the capacitor. A passivation layer is formed on the upper electrode 116 and surrounding areas. The passivation layer has an opening 120 that exposes a portion of the upper electrode 116. A pixel electrode layer 118 is electrically connected to the upper electrode 116 through the opening 120. Finally, other structural components of a liquid crystal display such as a color filter panel is assembled with the transparent substrate and a liquid crystal (not shown) is injected therein to form a liquid crystal display.
In the aforementioned LCD structure, the channel regions of most thin film transistors 104 are made using amorphous silicon(Si:H). During the patterning operation, some conductive residual material such as unwanted amorphous silicon material 115 may deposit along the edges of the capacitor lower electrode 114 and accumulate above the capacitor dielectric layer 124. Hence, in the fabrication of the so-called second metallic layer for forming the capacitor upper electrode 116 and the signal lines 112, the upper electrode 116 will cover and cross over the edges of the lower capacitor electrode 114 of the capacitor. If some of the conductive residual material 115 is retained on the capacitor dielectric layer 124, a short circuit between the capacitor upper electrode 116, the signal line 112 and the pixel electrode 118 will occur leading to pixel defects in the LCD array.
The presence of conductive residual material 115 may also lead to a short circuit between the upper and the lower capacitor electrode causing the storage capacitor 108 to malfunction. The conductive residual material 115 may be removed by shining a laser beam and burning out the material. However, the process may also break the normal line connection with the common electrode 114 and lead to a shallow line for the gate terminal. To prevent the formation of shallow lines, the defective capacitor is frequently not repaired so that the defective bright spot remains on the LCD.
Nevertheless, stringent demand for high quality image in the market is a major force for the use of laser to repair bright spot and attain a zero bright spot target. At present, laser repair technique has not progressed far enough for spot darkening to be carried out as routine. This is because the common electrode and the gate terminal may form a short circuit after the repair and result in a bright line defect. Thus, a method capable of repairing storage capacitor point defect and at the same time permitting the execution of spot darkening operations is needed for improving image quality.